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CVE-2015-2721: NSS incorrectly permits skipping of ServerKeyExchange

Mozilla Network Security Services (NSS) before 3.19, as used in Mozilla Firefox before 39.0, Firefox ESR 31.x before 31.8 and 38.x before 38.1, Thunderbird before 38.1, and other products, does not properly determine state transitions for the TLS state machine, which allows man-in-the-middle attackers to defeat cryptographic protection mechanisms by blocking messages, as demonstrated by removing a forward-secrecy property by blocking a ServerKeyExchange message, aka a “SMACK SKIP-TLS” issue.

CVE
#mac#perl#firefox#ssl

Mozilla Foundation Security Advisory 2015-71

Announced

July 2, 2015

Reporter

Karthikeyan Bhargavan

Impact

Moderate

Products

Firefox, Firefox ESR, Firefox OS, SeaMonkey, Thunderbird

Fixed in

  • Firefox 39
  • Firefox ESR 31.8
  • Firefox ESR 38.1
  • Firefox OS 2.2
  • SeaMonkey 2.35
  • Thunderbird 31.8
  • Thunderbird 38.1

Description

Security researcher Karthikeyan Bhargavan reported an issue in Network Security Services (NSS) where the client allows for a ECDHE_ECDSA exchange where the server does not send its ServerKeyExchange message instead of aborting the handshake. Instead, the NSS client will take the EC key from the ECDSA certificate. This violates the TLS protocol and also has some security implications for forward secrecy. In this situation, the browser thinks it is engaged in an ECDHE exchange, but has been silently downgraded to a non-forward secret mixed-ECDH exchange instead. As a result, if False Start is enabled, the browser will start sending data encrypted under these non-forward-secret connection keys. This issue was fixed in NSS version 3.19.1.

References

  • NSS incorrectly permits skipping of ServerKeyExchange (CVE-2015-2721)
  • SMACK: State Machine AttaCKs

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